This invention relates to iron ore pellets containing coarse ore particles, and more particularly to iron ore pellets which can be produced with a high pelletizing efficiency and which show excellent reducibility at high temperatures and physical strengths.
Iron ore pellets were originally developed as a technique for refining low grade ores, producing iron ore moldings suitable as a feed material of a blast furnace after grinding and sorting low grade iron ores into a powdery form with an increased iron content. However, the ore pellets have come to have a greater significance even to ores of higher grades for effectively utilizing the fine ore powder which occurs in mining and subsequent sintering stages.
In most cases, the conventional iron ore pellets are produced by pelletizing and sintering raw ore material of finely ground powdery form more than 70 to 90 wt % of which have a particle size smaller than 325 mesh (about 0.04 mm). This is because a high percentage of coarse particles in the pelletizing material narrows the range of moisture content which is suitable for pelletization, lowering the efficiency of pelletization and resulting in green pellets with considerably poor physical strengths (especially dropping strength). Ore pellets which are formed from a material containing fine ore particles in a large proportion are superior in strengths after sintering and low temperature reduction but have a great difficulty in practical applications in that they are inferior in reducibility at high temperature which is the most important property to a feed material to a blast furnace.
With the foregoing in view, the present inventors have conducted extensive studies for the purpose of improving the reducibility of iron ore pellets at high temperatures, and as a result found that iron ore pellets obtained by pelletizing and sintering fine ore powder containing 25 to 40 wt % of coarse particles of 0.1 mm or greater in diameter have improved reducibility at high temperatures. The pellets which are formed from fine ore powder containing a suitable amount of coarse ore particles have bridge-like slag bonds formed by self-fluxing fine particles between the individual coarse particles and contain an increased number of open pores. Therefore, the pellets are free of metal iron shells which are the main cause of low reducibility, and the metal iron is formed even in the inner regions of the individual pellets. As a result, the quantity of wustite which produces low melting point slag is reduced, the open pores become less susceptible to clogging and the softening contraction at high temperatures is lowered. In addition, upon softening under loaded high temperature conditions, the coarse ore particles play a role of an aggregate which lessens the deformations at high temperatures, ensuring excellent reducibility at high temperatures. In a case where the coarse ore content is limited to about 40%, the lowering in pelletizability and strengths of the pellets is prevented to some extent although the values are apparently lower than those of the pellets which are produced from fine ore powder alone.